The present invention relates to a process for producing mannitol-rich solutions of sorbitol and mannitol. More particularly, this invention relates to a process for producing a mannitol-rich solution of sorbitol and mannitol from glucose.
It is well known that a mixture of sorbitol and mannitol in aqueous solution can be produced by catalytic hydrogenation of invert sugar, which is an approximately equimolar mixture of glucose and fructose. Invert sugar, in turn, is commonly obtained by inversion of sucrose (ordinary sugar). The yield of mannitol is ordinarily about 24-26 percent by weight, based on total dry solids, when hydrogenation is carried out under neutral or mildly acidic conditions, such as those disclosed in U.S. Pat. No. 2,759,024 to Kasehagen. This yield can be increased by carrying out at least part of the hydrogenation under alkaline conditions, as described in U.S. Pat. Nos. 3,329,729 to Brandner et al and 3,763,246 to deBerardinis, or by appropriate choice of catalyst, as described in U.S. Pat. No. 3,705,199 to deBerardinis, or both. The processes of U.S. Pat. Nos. 3,329,729 and 3,763,246 are plural stage processes in which alkaline hydrogenation is followed by acid hydrogenation. Alkaline agents for the alkaline hydrogenation stages of those processes are alkali metal hydroxides such as sodium hydroxide, and alkaline earth metal oxides and hydroxides such as lime. U.S. Pat. No. 3,329,729 also suggests the addition of calcium carbonate as a buffering agent in addition to lime. Mannitol yields (in percent by weight on the dry basis) are as follows: U.S. Pat. Nos. 3,329,729, 30-36%; 3,705,199, 28-29%; 3,763,246, 27-31%. In each case the balance of the reaction product is mostly sorbitol.
Enhanced yields of mannitol under alkaline hydrogenation conditions are due to isomerization of part of the glucose present to fructose and mannose. The proportions of glucose, fructose, and mannose in the reaction mixture will vary depending on the alkaline material and the conditions used, and significant quantities of mannose are not ordinarily obtained. Such isomerization is well known in the art, and is discussed, for example, in U.S. Pat. Nos. 3,329,729 and 3,763,246 cited supra, and in Pigman, "The Carbohydrates: Chemistry, Biochemistry, and Physiology," Academic Press, New York 1957, pages 60-69.
High cost of mannose and fructose in substantially pure form preclude the economic use of these sugars as starting materials, even though mannose yields essentially pure mannitol and fructose yields a 50:50 mixture of sorbitol and mannitol on catalytic hydrogenation in neutral media.
Hydrogenation of either glucose or mannose in the presence of a platinum catalyst and considerable alkali yields a mixture of sorbitol and mannitol, according to U.S. Pat. No. 1,990,245 to Mueller et al. This reference also states, however, that pure sorbitol is obtained when glucose is hydrogenated at a pH from 7 to 12 in the presence of a nickel catalyst. The reaction medium is made alkaline by the addition of an alkali or alkaline earth metal oxide, hydroxide, or alkaline reacting salt such as a carbonate, silicate, or borate. Other references teach that hydrogenation of glucose in the presence of a nickel catalyst yields a mixture of sorbitol and mannitol under strongly alkaline conditions (pH 10 or over; British Pat. No. 1,129,586), while substantially pure sorbitol is obtained under milder alkaline conditions (pH 8.3-8.5; British Pat. No. 1,140,477). Canadian Pat. No. 735,972 to Silber describes catalytic hydrogenation of glucose and invert sugar to mixtures of sorbitol and mannitol in aqueous solution which is made alkaline with lime and optionally also with calcium carbonate. Glucose is hydrogenated substantially quantitatively to sorbitol under neutral or mildly acidic conditions; see U.S. Pat. Nos. 1,963,999 to Larchar and 2,280,975 to Power.
A process for obtaining sorbitol-mannitol solutions from glucose by first catalytically epimerizing glucose in an acidic aqueous solution containing at least 50% by weight of glucose to obtain an epimerizate of glucose and mannose, and then catalytically hydrogenating this epimerizate to obtain an aqueous solution of sorbitol and mannitol, is described in U.S. Pat. No. 4,029,878 to Kruse. Epimerization according to that process is carried out at elevated temperature in the presence of a hexavalent molybdenum catalyst, such as molybdic acid or an anion exchange resin in the molybdate form. Hydrogenation catalysts and conditions for hydrogenating the glucose-mannose epimerizate to a mixture of sorbitol and mannitol in that process are conventional. Ordinarily the epimerizate will contain about 30% (e.g., about 27-33%) of mannose on the dry basis, and the mol percentage of mannitol in the final product is also usually about 30%; that is, the mol percentage of mannitol in the final product does not differ significantly from the mol percentage of mannose in the epimerizate.
Production of sorbitol-mannitol mixtures in a process which comprises the successive steps of epimerizing glucose in aqueous solution containing a hexavalent molybdenum catalyst, treating the epimerizate with glucose isomerase enzyme under isomerizing conditions, and hydrogenating the resulting sugar mixture (a mixture of glucose, mannose, and fructose in aqueous solution) under non-alkaline conditions, is described in Japanese Patent Publication 51-75008. The product contains approximately 40% by weight (dry basis) of mannitol, balance principally sorbitol.
Mannitol may be recovered from aqueous solutions containing both sorbitol and mannitol by fractional crystallization, as described for example in U.S. Pat. No. 3,632,656.
Although yields of mannitol can be enhanced by hydrogenating either glucose or invert sugar under alkaline conditions rather than under neutral or mildly acidic conditions, quantities of impurities are also greater when alkaline conditions are used. There is a need for a hydrogenation process in which enhanced mannitol yields are obtained while at the same time the amounts of impurities are minimized.